This invention relates to optical filters for electromagnetic radiation and, in particular, to filters utilizing birefringent elements.
Recent interest in laser communications systems operating in scattering media has motivated the development of spectral filter structures capable of providing an extremely narrow bandwidth over a very wide field of view. A narrow bandwidth is necessary to reject unwanted background light and thereby achieve a satisfactory signal to noise ratio, while a large angular aperture is required to facilitate the receipt of as much of the scattered signal radiation as possible.
Attention in this research has increasingly focussed on employing the natural dispersion of materials to produce new filter concepts, including Christiansen-Bragg filters, dispersive birefringent filters, and filters using the zero crossing of birefringence in certain uniaxial crystals. Two basic versions of birefringent filters, known as Lyot-Ohman and Solc filters, are familiar in the art. These filters consist of one or more birefringent crystal plates placed between polarizers and operate by virtue of the interference of polarized light. To achieve polarization interference, such a filter is designed to introduce an appropriate phase retardation between the components of the filtered light polarized parallel and perpendicular to the optic axes of the crystal plates.
The field-of-view of the basic Lyot-Ohman and Solc filter designs is too restricted for modern communications systems. By selecting a crystal material exhibiting a strong dispersion in its birefringence, however, a filter can be designed with a vary narrow bandwidth capability while concurrently accommodating a wide field-of-view. Nevertheless, this approach is limited to filter operation in the spectral regime near an iso-index point or near the optical band edge. Thus, the passband of such a filter can be selected from only a relatively small wavelength region.
As a consequence of these limitations, existing filter designs cannot provide the requisite performance at all desired wavelengths. A need has therefore developed for a birefringent filter design which can be readily adjusted to provide a very narrow passband with a wide field-of-view over a broad range of the spectrum.